The invention concerns a method to clear minefields and an apparatus to use it. Together with a mine-detection method this leads to an automatic mechanism for land mine clearance.
The hidden land mines are an almost world-wide problem. Their number is estimated to be more than 110 million. The minefields sown during wars are often not cleared afterwards. This fact becomes understandable when you consider that one mine costs about 10 $, but its clearance costs about 200 $ up to 1,000 $ (cf. [7], p. 21).
To solve this problem, it is necessary to develop a method that clears minefields efficiently, quickly, cheaply and reliably.
A further main problem is the great risk to health of persons clearing the minefields. Statistically, 0.1% of the mines being cleared cause an accident. In Kuwait more than 80 people clearing minefields lost their lives as reported in the press. So the clearance of an evacuated minefield from a distance is advantageous, because there is no damage to men or material. Especially the anti-tank mines and the anti-helicopter mines often sown together with anti-personnel mines are a great risk for the vehicle respectively the airplane clearing the mines.
The purpose of the invention is a civil clearance of generally-hidden landbased explosive devices, for instance land mines, especially anti-personnel mines (so called APM""s). This means a clearance rate of more than 99.6% as demanded by the UN (cf. [6], p. 25).
This method works, even if the mine is a plastic mine or contains little metal. Fully metal-enclosed mines cannot be destroyed, but can be found with this method. Even other munitions, such as ammunition can be found and cleared.
The method consists in emitting specific concentrated high-power microwave radiation onto the minefield or onto the mine""s location. To locate the hidden mines the minefield is irradiated by low-power radiation and the back-radiation is received in the microwave and/or the infrared spectrum. Then specific concentrated high-power microwave radiation is used to burn down the mine.
In order to locate the mines you can also use other well-known detecting methods, for instance as described in U.S. Pat. No. 5,592,170 and other patents (see list) or Sieber et al. [4].
This method intends to destroy mines automatically from a distance.
The wavelength of microwaves is within the cm up to the dm band, that is frequencies in the SHF and EHF- band according to DIN 40015.
A land mine consists of a cover that is often made of plastic and no longer of metal, which is why their detection is difficult. The mine consists essentially of a fuse with the detonator and the explosive charge with a lower sensitivity to impact, friction and temperature-change.
Physically, the explosive has a dielectric loss Pd when irradiated by an electromagnetic wave, depending on the frequency f, the power of the electromagnetic wave, the dielectric number ∈r of the material and the dissipation factor tan(xcex4) of the explosive. So when irradiated by electromagnetic energy, the explosive of a not completely metal-enclosed mine absorbs part of the wave""s energy (cf. FIG. 4). The dielectric power loss Pd of the mine being directly exposed to the radiation is proportional to the product of the frequency f, the dielectric number ∈r and the dissipation factor tan(xcex4) of the explosive, whereas ∈r and tan(xcex4) also depend on the temperature and on the frequency f. (For highly polar materials such as water, the variation of the dielectric number ∈r with frequency is dramatic, going from 80 at 60 Hz to 20 at 10 kHz, because permanent dipols can no more follow the rapid field change.)
At various discrete values of temperature and frequency there are quasi-resonances where the loss is locally very high. A sharpe rise in dielectric loss, usually accompanied by a rapid temperature increase, is an indication of impending breakdown. If the absorbed power is high enough, the mine will burn down. The effect depends on the dissipated power inside the mine:
1) The explosive will gas out at lower temperatures and be defused,
2) the explosive will be heated above the deflagration point and burns down, or detonates when heated above the fusion point,
3) the explosive detonates immediately because the electromagnetic effect leads to electrical breakdown of the explosive depending on it""s dielectric strength.
The deflagration points and the fusion points of most explosives (as pure substances) are between 300 F. and 720 F. (cf. [5]).
If the electric field irradiating the explosive is increased to some critical magnitude, depending on the dielectric strength of the medium, the temperature, the medium surrounding the dielectric and on other factors, the material abruptly becomes conducting, a large current flows and local destruction occurs.
This electrical breakdown leads to detonation of an explosive device which is not metal-enclosed.
A further point of the invention is the use of microwave frequencies between 3 GHz and 80 GHz, so that the dielectric loss inside the explosive is high enough and the radiation penetrates the soil deeply enough.
Especially Industrial-Scientific-Medical (ISM) frequency bands according to EN 55011 and VDE 0875 are of advantage because there is no interference with radio operators and because they can effectively be generated by gyrotron oscillators according to claim 11 (see also FIG. 1). Using the ISM-frequency band of 61 GHz up to 61.5 GHz another advantage is the relatively high absorption of the stray radiation by the air at this frequency to avoid scattered radiation (no window frequency). The frequency of radiation in the known ISM-frequency band is around 5.800 GHz; 24.125 GHz; and 61.250 GHz.
The advantages of a gyrotron device besides the coherent and bundled high-power radiation compared to a magnetron radiation at say 0.91 GHz or 2.45 GHz are:
1) Because of the shorter wavelength at the higher frequency, the microwave radiation generated by a gyrotron device can be deflected and focused by relatively small quasi-optical reflectors (see (5) and (5xe2x80x2) in FIGS. 1 to 4).
2) The absorbed power in the dielectric medium (here: the explosive in the mine) being directly exposed to the radiation increases with increasing frequency and dissipation factor which also grows with higher frequency. So, compared to 2.45 GHz the absorbed power at 61.25 GHz is about 400 times higher.
3) The higher frequency establishes a higher resolution as ground-penetrating radar.
A further possibility is the pulsed emission of microwave radiation even at a higher power level. This can be done by a gyrotron device. We suggest emitting more than one pulse on each mine.
According to claim 3 you get a frequency band which penetrates the soil deeply enough and is absorbed adequately by the explosive.
Even higher frequencies do not penetrate the ground deeply enough because of their short wavelength (i.e. the function P1 decreases with decreasing xcex).
Because microwaves in the frequency band of 3 GHz up to 80 GHz are well absorbed by dielectric substances and because gyrotrons are masers with high power output of more than 10 kW cw, and total efficiency xcex7 of more than 50%, whose coherent radiation can be deflected and focused by small quasi-optical reflectors, we suggest to clear landbased minefields by defusion or explosion of the hidden mines by high-power microwave radiation.
Especially the use of radiation at a ISM-frequency of 61.250 GHz has the advantage that the stray radiation will be well absorbed by the air (no window frequency), so the scattered radiation is low.
We suggest using a gyrotron oscillator with a permanent magnet and a water edge-cooled microwave transmission window. This equipment is easy to handle and of low volume. The invented apparatus will be provided with quasi-optical reflectors in order to reflect and focus the emitted microwave radiation according to claim 13.
Gyrotrons with kW power rating and frequencies around 30 GHz are lately used for technological applications to heat ceramics by dielectric loss in a closed cavity (cf. [1]), p. 32 or [2], p. 22). To use the maser for technical applications, you even can focus the radiation on an area of less than 0.2 square inch (cf. [8], p. 8).
Last but not least, the invention suggests to fit the microwave emitter in a vehicle or an airplane, for instance a helicopter. An electric power generator supplies the equipment with electric energy.
The electronics of the vehicle must be electromagnetically shielded against the stray radiation. In addition, the microwave equipment is shielded except for the radiation exit (see FIG. 3).